Online Source of Shortwave Broadcasts – Clean, easy interface to find listings by the hour
SWL Archive – Excellent source of current info on worldwide SWL stations and an archive of SWL broadcasts for several stations.
Your Guide to Shortwave Radio Listening, News and Reviews – A beginner’s guide
Wikipedia – Concise overview of Spy Numbers Stations. Read this to better understand the history of Spy Numbers. A numbers station is a category of shortwave radio stations characterized by broadcasts of formatted numbers, addressed to intelligence officers operating in foreign countries.
Wikipedia – Shortwave radio entry briefly touches on many aspects of shortwave listening.
SWL Nostalgia Site – Loaded with interesting articles and links.